The goal of this proposal is to determine the pathophysiology of the autosomal recessive disorder. Thiamine-Responsive Megaloblastic Anemia (TRMA). Patients with TRMA exhibit unique, vitamin-responsive anemia, as well as progressive sensorineural deafness and non-type I diabetes responsive in part to pharmacologic doses of thiamine. Fibroblasts from TRMA patients are sensitive to thiamine deprivation in culture, and undergo apoptotic cell death in thiamine concentrations less than 9 nM. Normal fibroblasts exhibit high affinity (nanomolar) thiamine uptake, whereas mutant cells do not. The gene defective in TRMA was mapped and cloned. It encodes a high-affinity thiamine transporter, a member of the solute carrier family. designated SLC19A2. In all known TRMA patients, homozygous defects in the SLCI9A2 gene have been identified. However, the basis of the disease phenotype remains obscure. The anemia of TRMA reveals both megaloblastic changes and ringed sideroblasts, reminiscent of acquired Myelodysplastic Syndromes. The bases of anemia, deafness and diabetes in TRMA are obscure. Our working hypothesis is that the defective thiamine transporter of TRMA results in intracellular metabolic changes responsible for features of the disease. The goals of the current proposal are: (1) To characterize the high-affinity thiamine transporter and its role in thiamine dependent metabolic pathways in TRMA and control cells in tissue culture. Synthesis and distribution of SLC19A2 protein will be studied, and metabolic pathways assessed by GC/mass spectroscopy analysis of stable isotope-tagged glucose metabolites. (2) To characterize high affinity thiamine transport and its role in mice, targeted disruption of the SLCI9A2 gene will be used to create an animal model for human pathophysiology. (3) To examine the role of thiamine transport system in hematopoiesis, using murine marrow from normal and TRMA animals. Clearer understanding of the role of the thiamine transporter will help to elucidate the basis of multi-organ system disease in TRMA, and improve fundamental understanding of the role of thiamine and thiamine transport in normal hematopoiesis.